1. Field of the Invention
The present invention relates to a digital data reproducing apparatus and method that include a digital interface for receiving digital data such as audio data from an external device independent of a main body of the apparatus and reproduce the digital data via the main body, and a storage medium storing a program for realizing the method, and also to a digital data transmitting apparatus and method that transmit digital data via the main body, y, and a storage medium storing a program for realizing the method.
2. Prior Art
There is known a system that includes a digital interface for receiving digital data such as audio data from an external device independent of a main body of the system, and records or reproduces the digital data received by the interface, in the main body. In this system, the digital data from the interface are processed using a clock signal which is synchronized with both the operation of the main body and that of the interface. Alternatively, in order to achieve synchronization in operation between the external device and the main body, a sophisticated digital signal processing circuit is used to perform sampling rate conversion on the digital data from the interface to realize the same sampling rate between the external device and the main body.
Such clock synchronization may be achieved in several ways. For example, a master clock signal may be output from the main body and supplied to the external device which drives an external digital source. Or, a phase locked loop (PLL) may be provided within the main body to generate a clock signal in synchronism with a clock signal applied to process the digital data input from the external interface so that the main body is driven using this clock signal. These processing methods cannot be carried out, however, unless a sampling rate employed by the external digital source is the same as that employed by the main body. When the two sampling rates are different, it is a general practice to process data themselves from the external digital source by performing sampling rate conversion on the data to thereby obtain the same sampling rate between the external device and the main body.
In the prior art, as described above, a PLL or an elaborate sampling rate conversion circuit is required to achieve the clock synchronization, leading to a significant increase in cost. Besides, the PLL cannot accommodate itself to data with different sampling rates. Also, conversion of the sampling rate by the sampling rate converter circuit makes it impossible to record and transmit digital data that are faithful to the original data.
The method of processing data themselves by sampling rate conversion is applicable to most conventional audio data, since the conversion of the digital data themselves does not give rise to any problem. However, this method cannot be applied to case where the digital data are encoded digital data such as compressed data, or to case where digital information data which, unlike audio-visual data, do not allow data conversion and are transmitted using an existing format at a sampling rate for audio data.
It is a first object of the present invention to provide a digital data reproducing apparatus and method that are capable of reproducing both digital audio data from a main body of the apparatus and digital data from an external interface, using a simple construction, and a storage medium storing a program for realizing the method.
It is a second object of the present invention to provide a digital data transmitting apparatus and method that are capable of recording and transmitting received digital data of any kind in a state faithful to the original data, using a simple construction without converting the data, and a storage medium storing a program for realizing the method.
To attain the first object, according to a first aspect of the present invention, there is provided a digital data reproducing apparatus comprising a main body that processes first digital data, an interface that receives, from an external device independent of the main body, second digital data having a sampling rate asynchronous with and different from a sampling rate of the first digital data and which have a different sampling rate, the first and second digital data being reproduced by the main body, a filter device that oversamples the first digital data at a frequency n times the sampling rate of the first digital data, an adder that adds together the first digital data oversampled by the filter device and the second digital data which are input via the interface, and a converter that converts digital data resulting from addition by the adder into an analog signal.
In a preferred form of the first aspect, the interface carries out zero-order holding of the second digital data and supplies the zero-order held data to the adder.
Alternatively, the interface carries out linear interpolation of the second digital data and supplies the linear interpolated data to the adder.
To attain the second object, according to a second aspect of the present invention, there is provided a digital data transmitting apparatus comprising a first interface that transmits first digital data at a first sampling rate, a second interface that transmits second digital data at a second sampling rate which is asynchronous with and independent of the first sampling rate, a main line control device that transmits in an intermittent manner the first digital data which are received from the first interface, together with a first flag. to the second interface using a main line, and an auxiliary line control device that is enabled transmits, when the first sampling rate is greater than the second sampling rate, for transmitting an overflow of the first digital data occurring due to transmission of the first digital data by the main line alone, together with a second flag, to the second interface using an auxiliary line.
In a preferred form of the second aspect, the digital data transmitting apparatus comprises a filter device that oversamples the second digital data received from the second interface at a frequency n times the second sampling rate, an adder that adds together the first digital data received from the first interface and the second digital data oversampled by the filter device, and a converter that converts digital data resulting from addition by the adder into an analog signal.
To attain the first object, according to a third aspect of the present invention, there is provided a digital data reproducing method comprising the steps of processing first digital data by a main body device, receiving, by means of an interface, from an external device independent of the main body, second digital data having a sampling rate asynchronous with and different from a sampling rate of the first digital data, the first and second digital data being reproduced by the main body device, oversampling the first digital data at a frequency n times the sampling rate of the first digital data, adding together the first digital data oversampled by the oversampling step and the second digital data which are input via the interface, and converting digital data resulting from addition by the adding step into an analog signal.
To attain the second object, according to a fourth aspect of the present invention, there is provided a digital data transmitting method comprising the steps of transmitting, by means of a first interface, first digital data at a first sampling rate, transmitting, by means of a second interface, second digital data at a second sampling rate which is asynchronous with and independent of the first sampling rate, transmitting in an intermittent manner the first digital data which are received from the first interface, together with a first flag. to the second interface using a main line, and transmitting an overflow of the first digital data occurring due to transmission of the first digital data by the main line alone, together with a second flag, to the second interface using an auxiliary line when the first sampling rate is greater than the second sampling rate.
To attain the first object, according to a fifth aspect of the present invention, there is provided a storage medium storing a program which can be executed by a computer, for realizing the above described digital data reproducing method.
To attain the second object, according to a sixth aspect of the present invention, there is provided a storage medium storing a program which can be executed by a computer, for realizing the above described digital data transmitting method.
According to the first, third, and fifth aspects of the present invention, the first digital data obtained by oversampling with the filter device at a frequency n times the sampling frequency thereof, and the second digital data that are input via the interface are directly added together by the adder. This allows the second digital data having a sampling rate quite different from that of the first digital data to be transmitted with their characteristics unchanged to be digital-to-analog converted. This enables digital audio data both from the main body and from the external device to be reproduced using a simple construction.
According to the second, fourth, and sixth aspects of the present invention, the first digital data input from the first interface are intermittently transmitted together with the first flag to the second interface, using the main line by the main line control device. On the other hand, when the first sampling rate is higher than the second sampling rate, an overflow of the first digital data that occurs due to the transmission of the first digital data via the main line is transmitted together with the second flag to the second interface using the auxiliary line by the auxiliary line control device. This enables digital data of any kind to be recorded and transmitted in a state faithful to the original data without converting the received digital data, using a simple construction